This invention generally relates to optical amplifiers, and is specifically concerned with a hybrid Raman-erbium doped fiber amplifier (EDFA) that achieves a smaller maximum noise figure across its output spectrum than an individual EDFA of comparable gain.
Erbium-doped fiber amplifiers (EDFAs) are used in optical transmission networks to extend transmission distances and to compensate for losses from various network elements. Such amplifiers typically comprise at least one pump laser whose output is optically coupled to the input of one or more serially connected coils of erbium-doped optical fiber. In operation, the output of the pump lasers excites the atoms of the erbium dopant within the serially connected coils. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output. While a single EDFA may be used in an optical network, an EDFA assembly comprising a pair of EDFAs connected by a dispersion-compensating module (DCM) is preferred as such an assembly provides higher gain with less dispersion-type distortion than a stand-alone EDFA.
Raman amplifiers are also used in optical transmission networks for the same purposes as EDFAs. In lieu of erbium-doped optical fiber, Raman amplifiers advantageously use standard optical fiber in achieving amplification. In such an amplifier, the output of a pair of orthogonally polarized pump-diode lasers provide backward propagating pump power in the transmission fiber. Forward-propagating signals achieve gain in the fiber because higher-energy (shorter wavelength) pump photons scatter off the vibrational modes of the optical fiber""s lattice matrix and coherently add to the lower-energy (longer wavelength) signal photons. While the maximum gain levels that can be achieved with such Raman amplifiers are typically less than those achievable by EDFA amplifiers, Raman amplifiers are more economical since they require no specially doped optical fiber and can act as a low noise pre-amplifier before the EDFA.
In order to maximize the data through-put of an optical network, different optical signals are transmitted over different channels across a discrete portion of the optical spectrum. Such an optical spectrum may be, for example, the L-band window, which includes wavelengths of between 1570 to 1620 nm.
Ideally, the optical amplifiers in such networks should be able to amplify each channel within a selected transmission spectrum to the same level of gain. Stated differently, an ideal optical amplifier has a flat or uniform gain for all channels present across the output spectrum of the network. Such a flat gain output minimizes signal information losses throughout the network. By contrast, when the gain of an optical amplifier is non-uniform, signal information can ultimately be lost because of the progressive under-amplification of certain channels as they are propagated through the network. Ultimately, the inherent noise component present in such under-amplified channels overtakes and destroys the signal information.
One of the shortcomings of EDFAs is their non-flat gain characteristics across a given optical spectrum. In particular, the gain level is substantially less at the xe2x80x9credxe2x80x9d end of the L-band between about 1600 and 1620 nm. Fortunately, the gain level of such amplifiers can be rendered substantially flat (i.e., with only about a 4% change at a 25 dB gain level) across the L-band window by the use of gain flattening filters which are optically coupled between the coils of erbium doped fiber. Unfortunately, the use of such filters results in a higher noise figure in the channels having wavelengths in the 1600-1620 nm range as illustrated in FIG. 1A. Here, the noise figure typically increases by approximately 33%. By contrast, the change in the noise figure in the channels within the 1570-1600 nm range of the spectrum is only about 3%. The substantially higher noise figure in the 1600-1620 nm range lowers the usable bandwidth available from such EDFA amplifiers.
Raman amplifiers likewise have non-flat gain characteristics. As illustrated in the graph of FIG. 1B, a typical Raman gain level curve has minimums at about 1570 nm, 1595 nm, and 1620 nm, and maximums at 1585 nm and 1610 nm. Gain can vary about 13% between the maximum and minimum points in the FIG. 1B example. A gain flattening filter can be applied to reduce this variation but will only be optimized at a single operating gain value. Additionally, there is the desire to minimize the number of gain flattening filters in the system and the loss they incur.
Clearly, there is a need for a bray to reduce the maximum noise figure in EDFA gain, as well as to further flatten the gain curve in Raman-type amplifiers in order to reduce signal losses throughout the network. Ideally, such noise figure reduction and gain flattening should be achieved without the use of additional gain flattening filters, which operate by reducing the gain of the most amplified channels to a level consistent with the lowest amplified channels throughout the output spectrum. Finally, it would be desirable if such noise figure reduction and gain flattening could be easily applied to optical amplifiers already in use in existing networks without the need for extensive modifications or new and expensive components.
The invention is a hybrid optical signal amplifier that reduces the maximum noise figure of an EDFA while flattening the gain of a Raman amplifier without compromising laser pump efficiency. To this end, the hybrid optical signal amplifier of the invention comprises a Raman amplifier having a gain level that varies across a spectrum of optical wavelengths and defines regions of higher and lower gain levels; an EDFA assembly having an input connected to an output of the Raman amplifier, and an output with a noise figure that varies across the same spectrum of optical wavelengths and defines a region that includes a maximum noise figure, wherein a spectrum region of higher gain level of the Raman amplifier corresponds to the spectrum region of the EDFA assembly that includes the maximum noise figure. The combination of a Raman amplifier and an EDFA assembly having such gain output characteristics advantageously reduces the maximum noise figure of the final output while providing a gain having less ripple than a stand-alone, ripple optimized Raman amplifier.
While the invention encompasses the concept of an EDFA assembly in combination with a Raman amplifier which has had its output ripple individually optimized, a more preferred embodiment of the invention is the combination of an EDFA assembly and a Raman amplifier whose gain ripple has been adjusted to maximally reduce the region of highest noise figure in the output spectrum of the EDFA assembly to optimize the output of the hybrid amplifier. To this end, either or both of the EDFA assembly or the Raman amplifier may includes an optical filter for optimally flattening the gain spectrum of the hybrid optical signal amplifier.
The EDFA assembly may include a pair of EDFAs serially connected via a dispersion compensating module. Such an arrangement efficiently provides a higher level of distortion free gain over the output spectrum than the use of a single stage EDFA.
In the preferred embodiment of the hybrid optical signal amplifier, the magnitude of the maximum noise figure is less than 2.0 dB, and even more preferably less than 1.5 dB. Additionally, the wavelength spectrum for both the gain and noise figure is preferably between 1400 and 1750 nm, and more preferably in the L-band between about 1570 and 1630 nm.
The invention provides a higher and more noise-free and less ripple distorted output than is normally achieved by the use of either a stand-alone EDFA assembly or Raman amplifier.